This invention relates to cation exchange resins. More specifically, it relates to a method of improving the stability of cation exchange resins, and the cation exchange resins prepared from this method.
Cation exchange resins are useful for such applications as water treatment, for example, water softening and water deionization for powerplant boilers (often referred to as "condensate polishing"); chemical purification of food and pharmaceutical products by chromatographic separation, for example, separating fructose from glucose in the production of high fructose corn syrup; and catalysis. Unfortunately, the resins decompose over time and release organic and inorganic impurities into the process stream. Measurable decomposition products may contaminate the process stream and prevent continued use of the resin for a given application.
The decomposition of cation exchange resins is caused by oxidation of the crosslinked, copolymer matrix. The copolymer matrix breaks up as the polymer chains fragment to form various decomposition products, which are described in Stahlbush et al., "Prediction and Identification of Leachables from Cation Exchange Resins", Proceedings of the 48th International Water Conference (held Nov. 2-4, 1987). In addition to introducing decomposition products into the process stream, the decomposition of the resin may also adversely affect its chemical and physical properties. For example, the wet-volume capacity and the crush strength of the resin may be reduced.
The rate of release of decomposition products from the resin can vary significantly depending on the amount of crosslinking present in the copolymer matrix and the ionic form of the resin. Generally, the rate of release increases as the level of crosslinking decreases. The hydrogen form of the resin is typically more stable than other ionic forms of the resin, such as the calcium, sodium or ammonium forms of the resin. Therefore, resins in ionic forms other than the hydrogen form and with low levels of crosslinking are most susceptible to unacceptably high rates of decomposition. Unfortunately, resins with these characteristics are often required for numerous applications. For example, a cation exchange resin in the calcium form with low levels of crosslinking is the preferred resin for the chromatographic separation of fructose from glucose in the production of high fructose corn syrup. However, even resins with minimal rates of decomposition may be unsuitable for certain applications if trace amounts of decomposition products in the process stream cannot be tolerated.
Attempts have been made to improve the oxidation resistance of cation exchange resins. U.S. Pat. No. 3,342,755 discloses halogenating the crosslinked copolymer matrix before functionalizing the resin to increase oxidation resistance. Unfortunately, large amounts of halogenating agents are required for significant improvements.
In view of the lack of an available method in the prior art to increase the stability of cation exchange resins, a method of improving the stability of a cation exchange resin is needed.